Recently, the need for high-capacity rechargeable batteries has been increasing with the miniaturization of electronic devices. In particular, nonaqueous solvent-type lithium rechargeable batteries are attracting public attention for their energy density which is higher than that of nickel-cadmium batteries and nickel-hydrogen batteries. Although the capacity of lithium rechargeable batteries has been increasing, nowadays a much higher capacity is required as the performance that batteries are expected to have is getting higher.
As a negative electrode material for such a lithium rechargeable battery, graphite and other materials have been investigated. Graphite has been conventionally used because of its excellent charge/discharge cycle properties, suppressed electrode expansion in volume and low cost. However, a negative electrode material derived from graphite has a limitation in that the maximum theoretical capacity thereof is 372 mAh/g and thus cannot be further improved in capacity. Therefore, research focus is shifting from graphite negative electrodes to alloy-type negative electrodes containing a metal such as Si, Sn and Al, which forms an alloy together with lithium having a high theoretical capacity. Si has an especially high capacity and the application thereof as a negative electrode has been widely attempted. However, an Si-based negative electrode markedly expands in volume during the reaction with lithium, thereby posing problems that Si is pulverized or removed from a current collector, the reactivity thereof with an electrolyte solution is high, and the charge/discharge cycle properties are poor. This leads to a need for a novel negative electrode that has the advantages of the alloy-type negative electrode and exhibits suppressed reactivity with an electrolyte solution, excellent charge/discharge cycle properties and a weaker tendency to expand in volume.
Patent Document 1 described below states that heat treatment of Si particles and a carbon precursor provides a negative electrode material that consists of Si and C and contains little or no SiC, and that the absence of SiC having no capacity allows the negative electrode material to have a high capacity and excellent charge/discharge cycle properties.
Patent Document 2 described below states that, when partially nitrided by heat treatment under nitrogen atmosphere, particles of silicon oxide form SiNxOy particles, which provide a lithium rechargeable battery excellent in both capacity and charge/discharge cycle properties.
Patent Document 3 described below states that the silicon particles containing a small thermodynamically-estimated amount of silicon boride such as SiB4 and 0.1 to 50 wt % of substantially supercooled boron provide a lithium rechargeable battery that exhibits a high capacity and an initial charge/discharge efficiency and excellent charge/discharge cycle properties.
Patent Document 4 described below states that the use of a thin-film electrode obtained by forming a thin film of Si or another similar element on a copper film substrate via vapor deposition or sputtering results in a lithium rechargeable battery that has a low electric resistance and high current-collecting properties as well as excellent voltage, capacity and charge/discharge properties.    Patent Document 1: Japanese Unexamined Patent Application Publication No. H11-339796    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2002-356314    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2000-149951    Patent Document 4: Japanese Unexamined Patent Application Publication No. H11-135115
Along with the growing need for higher-capacity batteries these days, the application of an Si-based negative electrode material, a high-capacity material, is increasingly desired. However, such an Si-based negative electrode material has problems that:
the increase in irreversible capacity and the consumption of lithium contained in a positive electrode active substance caused by the reaction with an electrolyte solution result in a lowered battery capacity;
pulverization and removal from a current collector of Si caused by the expansion and contraction in volume due to the insertion and removal of lithium atoms degrade the charge/discharge cycle properties;
the decrease in the amount of an active substance capable of occluding and releasing electrons due to the reaction with an electrolyte solution during charge/discharge cycles also degrades the charge/discharge cycle properties; and
electrode expansion in volume due to the insertion of lithium atoms accumulates during charge/discharge cycles, thereby increasing the battery volume, in other words, reducing the battery capacity per unit volume.
Consequently, besides the use of an Si-based active substance for improving the capacity, suppressing the reaction with an electrolyte solution, enhancing the charge/discharge efficiency at an initial stage and during charge/discharge cycles, improving the charge/discharge cycle properties and preventing the increase in electrode expansion in volume after charge/discharge cycles have been performed are strongly demanded to further improve the capacity of a lithium rechargeable battery.
However, the composite negative electrode material consisting of Si particles and carbon atoms disclosed in Patent Document 1 cannot improve the charge/discharge cycle properties sufficiently. This is because the composite material is made through heat treatment of a carbon precursor and thus the Si particles and carbon atoms exist in separate portions in the composite material, so that in the portions in which the Si particles are present, it is difficult to prevent the expansion in volume and pulverization of Si caused by discharge and charge.
In Patent Document 2, particles of the partially nitrided silicon oxide SiNxOy are formed using SiOz as material, and thus the content of oxygen, which may react with Li, is large. This reduces the charge/discharge efficiency at an initial stage, thus leading to the problem that the battery capacity is limited.
As for Patent Document 3, the disclosed material, which is made by dispersing boron atoms in Si containing a small amount of silicon boride, expands in volume during charge because of the reaction of the boron atoms themselves with Li. This results in insufficient prevention of the interruption of conductive paths between Si atoms and current collectors, thus posing problems concerning the charge/discharge cycle properties.
In Patent Document 4, such interruption of conductive paths between Si atoms and current collectors is avoided by forming a thin film of Si via vapor deposition or sputtering, and thus the charge/discharge properties are improved. However, the reactivity between Si and an electrolyte solution is not suppressed, so that the charge/discharge cycle properties are insufficient. Furthermore, the use of a time-consuming film-forming method such as sputtering raises a productivity issue as well.